1. Field of the Invention
The present invention relates to a sample-and-hold circuit for generating a variable sample delay time of a transformer and a method thereof, and particularly to a sample-and-hold circuit for generating a variable sample delay time of a transformer and a method thereof that can generate the variable sample delay time varied with a load coupled to the transformer.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a diagram illustrating a transformer 100 according to the prior art. A control circuit 102 of a primary side of the transformer 100 is used for detecting a voltage of an auxiliary winding 104 to control an output voltage VOUT. Because the voltage of the auxiliary winding 104 is a positive voltage and a negative voltage in turn, the control circuit 102 needs to sample the voltage of the auxiliary winding 104 in a correct time. Generally speaking, a sample-and-hold circuit 108 generates a sample signal SS to a comparator 107 of the control circuit 102 after a switch 106 of the primary side of the transformer 100 is turned off for a period of time, where VREF is a reference voltage. However, because discharge times of the transformer 100 corresponding to different loads are different when the different loads coupled to a secondary side of the transformer 100, a stabilization effect for the output voltage VOUT of the control circuit 102 may be not good, and further the control circuit 102 samples a wrong voltage of the auxiliary winding 104 when a light load coupled to the secondary side of the transformer 100 if the sample-and-hold circuit 108 utilizes a fixed sampling time to sample the voltage of the auxiliary winding 104.